The present invention relates to a flexible-disc driving system which drives and rotates a flexible-disc to and from which information signals are written in and read out by means of a magnetic head.
The flexible-disc, sometime being called floppy disc, is a magnetic disc made of a polyester plate of about 80 .mu.m thick both sides of which are coated with a layer of magnetic iron oxide and which is encased in a cover about 20 cm square made of vinyl chloride or any other similar material and which is used for an information memory system as a peripheral device of electronic computers.
A patent disclosure dealing with the data storage apparatus employing a single magnetic disc is found in U.S. Pat. No. 3,678,481. A conventional system is outlined hereunder as illustrated in FIG. 1. In FIG. 1, 1 is the flexible-disc (hereinafter simply called disc), 2 is the driving hub, 3 is a centering cone and 4 is the driving axle, while the disc 1 is attached on the top of the driving hub by the centering cone in a detachable manner and the driving hub 2 is directly connected to the driving axle coaxially, wherein these elements 1, 2, 3 and 4 are designed to rotate together in one piece.
The driving axle 4 is driven by the driving motor 7 by way of pulleys 5-1 and 5-2 and belt 6. The magnetic head 8 for write-ins and read-outs and the pad 9 are arranged in a position so as to hold the disc 1 in between them. It is so structured that write-ins and read-outs are made by the magnetic head 8 and the pad 9 which are pressed against the disc 1 and in contact with the recording surface when the actuator (no drawing attached) that consists of a solenoid and other parts is excited by way of receiving head load instructions. The magnetic head 8 can also be driven back and forth toward the center of the disc 1 by means of a mechanism composed of the lead screw 10 and the stepping motor 11, whereby access to the recording track on the surface of the disc 1 is being made, 12 is a protecting cover of the disc 1 made of vinyl chloride which is, though not indicated in the drawing, held in a fixed position while the disc is rotating and has an oblong opening for write-in and read-out of information as well as for access to the recording track.
Generally, for the driving motor 7 an alternating current synchronous motor is employed, and is set to rotate the disc 1 at a given speed (e.g. 360 rpm).
The conventional system with the above structure, however, involves various problematic points as will be dealt with in the following. Although the disc 1 has to maintain an even revolution at a given rotation speed to improve the reliability in the behavior of write-in and read-out of information, the driving system which comprises an A.C. synchronous motor as well as a belt and pulley arrangement possesses the following shortcomings. First, as an A.C. synchronous motor rotates at a revolution speed proportionated to the frequency of the power source, its speed varies in accordance with the variation of the power source frequency. Next, in order to secure interchangeability between each driving system or each disc to which information has been written in, the revolution speed of the disc of each driving system is required to fall within a limited bracket of certain deviation values, so that a high processing accuracy is required for the belt and pulleys. In addition, in case of a manufacturer who manufactures these systems for districts where different power source frequencies are prevailing, he may have to make the sets of this system for each different frequency, or may have to produce the system in a way which facilitates replacement of the belt and pulleys that fit to the designated frequency. Furthermore, on account of the voltage difference by countries, on the part of the voltage difference by countries, on the part of manufacturers, separate motors which fit to each different voltage are to be provided.
Observing the uneven revolution characteristics next, the conventional system has the following inconveniencies. The variation in the load such as the frictional resistance between the disc 1 and the cover 12 as well as the frictional resistance at the contacting points of the magnetic head 8 and the pad 9 with the disc 1, and the instantaneous variation of the power source voltage give a transient variation to the revolution speed of the disc 1, namely an uneven revolution. The causes of an uneven revolution are, other than the above variation in load, asymmetrical winding of the synchronous motor, an uneven gap of the magnetic circuit due to a lack of uniformity in slots and magnetic anisotropy of the rotor. In general, the uneven revolution which is caused by a sharp up or down of the load torque of a synchronous motor results a damped oscillation. The driving system which employs a belt and pulley arrangement also is a great cause for vibration. As a means to minimize uneven revolution, a flywheel is usually employed in order to maximize the moment of load inertia against the motor, but it accompanies such disadvantages as an increased cost and weight for the flywheel and an extended time required for start-up. In this case, the range of the speed variation gets narrower though, the damping time gets longer. As other ways to minimize uneven revolution, synchronizing torque of the synchronous motor could be made larger, or viscous resistance of the driving system could be increased, but any of these adjustments results in such disadvantages as a larger size of the motor or wide changes in the basic characteristics of the motor itself.